The spatial distribution, including the kind and quantity of atmospheric pollutants in the vicinity of urban complexes or otherwise has been difficult to evaluate. These pollutants generally comprise an aerosol, which may be defined as a colloidal suspension of liquid or solid particles in the air.
The visual quality of air, including visual range and color, is related to air pollution by an atmospheric aerosol. Increases in atmospheric aerosol provide a degradation in visual quality which manifests itself in reduced visibility through the atmosphere.
In order to have an exact criterion of visual quality and to be able to measure the kind and quantity of atmospheric aerosol, the extinction coefficient due to light scattering may be measured.
The light scattering coefficient is defined in "A New Instrument for Evaluating the Visual Quality of Air" by Ahlquist and Charlson, Journal of the Air Pollution Control Association, Volume 17, Number 7, July 1967. The light scattering coefficient may be determined by implementing the following attenuation equation: EQU I/I.sub.o = e.sup.-.sup.bx
where:
I.sub.o = intensity of light prior to entering an atmospheric path. PA1 I = intensity of light after passage through an atmospheric path of distance x. PA1 b = extinction coefficient. PA1 b.sub.abs = extinction coefficient due to absorption of light by particles or gases. PA1 b.sub.scat = extinction coefficient due to scatter of light by particles or gases.
The quality b, is made up of two components, EQU b = b.sub.abs + b.sub.scat
where:
In general, it is possible to assume that b = b.sub.scat unless, of course, substantial quantities of light absorbing gases (e.g., NO.sub.2) or particles (e.g., soot) are present. For most atmospheric aerosols, b.sub.scat is estimated to be about 3 to 100 times larger than b.sub.abs. Integrating nephelometers have been built for measuring the scattering component of extinction coefficient, b.sub.scat. Such devices optically integrate light scattered in all angles by the aerosol particles to obtain a value of b.sub.scat and are to be distinguished from devices which measure light scattering only at a discrete angle or angles.
The term "scattering angle" as used hereinafter is defined as the angle included between the line projected from a light source and extending beyond a particle, and a line comprising the direction of light from the source which is scattered by the particle receiving light from the particle. A scattering angle of 0.degree. is defined as transmitted light, and a scattering angle of 180.degree. is defined as reflected light. The terms "light scatter," "total scatter," and "light scattering" as used hereinafter are thus defined as that measured by taking the integral of scattered light over all scattering angles.
The term "back scatter" as used hereinafter is defined as that measured by taking the integral of scattered light over scattering angles in the range of 90.degree. to 180.degree..
The advantage of the nephelometer over other devices for measuring the visual quality of air is that the nephelometer takes a reading of the coefficient b.sub.scat at what may be considered a point in space and in time. Since the integrating nephelometer may be designed to provide an output proportional to the extinction coefficient due to light scattering b.sub.scat, it is then a simple matter to relate this coefficient to spatial distribution of atmospheric aerosol by taking a plurality of readings at different points in space and time. It has been shown that the extinction coefficient due to light scattering can also be simply related to both visual range and to the mass of aerosol per volume of air by simple formulas.
An integrating nephelometer described in U.S. Pat. No. 3,563,661, Feb. 16, 1971, by Robert J. Charlson and Norman C. Ahlquist, which is also assigned to the assignee of the present invention, is particularly adapted for highly sensitive, accurate and correlatable monitoring of atmospheric aerosol. This device provides an electrical output signal which is proportional to the measured value of the extinction coefficient due to light scattering, or b.sub.scat, for a given aerosol sample.
The integrating nephelometer described in the aforementioned Charlson et al patent includes a sample chamber, an inlet and an outlet for introducing and removing a sample of the aerosol into and from the chamber, a light source which produces a light pulse of high intensity and having a cosine emission characteristic, and means situating the light source so that the maximum of the cosine emission characteristic is positioned at right angles to the axis of a cone of observation. A measuring photomultiplier tube is situated in the housing to receive the portions of the light pulse which are scattered by the aerosol sample and which pass through the vertex of the cone of observation. A reference photoelectric detector receives light directly from the source, and a divider circuit takes the ratio of the output from the measuring photomultiplier tube and the output of the reference detector. A trigger device and sampling means coordinates the production of the light pulse from the light source and sampling of the output from the divider circuit to provide a signal proportional to the desired extinction coefficient b.sub.scat.
The combination of a light source of high intensity such as a flashlamp, plus pulsed operation thereof, when combined with use of both a measuring and reference photodetector, more commonly referred to as a two-beam optical system, provides an improvement in sensitivity over previous integrating nephelometers by a factor of approximately 20, resulting in a detection limit for b.sub.scat of less than 1 .times. 10.sup.-.sup.5 m.sup.-.sup.1 (per meter). In addition, an instrument constructed according to the preferred embodiment of the Charlson et al patent is rugged and readily adaptable to hostile environments. Because of its versatility and sensitivity, this prior integrating nephelometer is adaptable to obtaining information about atmospheric aerosols which was heretofore unrecognized or unmeasurable by previous instruments.
An integrating nephelometer of this type lends itself to classification of aerosols by measurement of the extinction coefficient due to light scattering. In addition, such a nephelometer is adaptable to instantaneous measurements on aerosol samples from which the present characteristics thereof which are related to air pollution, such as visual range and mass concentration, can be determined, and from the future characteristics or effects of the aerosol upon the atmosphere can be predicted.
An instrument constructed according to the teachings of the above-identified Charlson et al patent has proved disadvantageous in some respects, however. For example, the circuitry involved is complex and requires many components, thereby resulting in a fairly expensive device. The flashlamp used in most embodiments is also an expensive item, and has proven to present problems with respect to instrument stability because of variations in flashlamp intensity. While the two-beam optical system and the timing and sampling techniques used in the instrument compensate for most of the short-term variations in flashlamp intensity, and additionally provide noise reduction so as to increase the instrument sensitivity as previously described, they have not proven adequate to compensate for long-term drift in the instrument. In addition, short-term noise resulting from variations in gain and changes in dark current of the measuring photomultiplier tube has limited the sensitivity and stability of the instrument.
An instrument constructed according to the preferred embodiment of the Charlson et al patent utilizes analog circuits, and, as such, has a restricted dynamic range. That is, in some instances, the extinction coefficient b.sub.scat might vary from 10.sup.-.sup.2 to 10.sup.-.sup.7 m.sup.-.sup.1 in a short time period and would be measureable only by changing the range sensitivity of the instrument.
Furthermore, the discontinuous operation of such an instrument, in which measurements are taken during only certain intervals of a measuring interval, decreases the signal-to-noise ratio over that obtainable, were continuous monitoring of the extinction coefficient b.sub.scat possible.
It is therefore an object of this invention to provide an improved integrating nephelometer which has a greatly extended dynamic range for the monitoring of the extinction coefficient due to light scattering of atmospheric aerosols.
It is another object of this invention to provide such an improved integrating nephelometer which is highly sensitive and which has an improved signal-to-noise ratio over previous integrating nephelometers.
It is yet another object of this invention to provide such an improved integrating nephelometer which is less expensive than previous integrating nephelometers.
It is still another object of this invention to provide such an improved integrating nephelometer which is capable of providing highly accurate measurement of the extinction coefficient due to light scattering of atmospheric aerosols at a plurality of wavelengths over an extended period of time.
It is a further object of this invention to provide such an improved integrating nephelometer which is self-calibrating at periodic intervals to further increase the sensitivity and stability of the instrument over previous integrating nephelometers.
It is yet another object of this invention to provide such an improved integrating nephelometer which can measure not only the extinction coefficient due to light scattering of atmospheric aerosols, but also the back scatter component of such extinction coefficient, at a plurality of wavelengths.
It is still a further object of this invention to provide such an improved integrating nephelometer which utilizes digital data processing techniques to effect extinction coefficient measurements.